Within a cell, the genetic material of an organism is packaged within the nucleus in long structures called chromosomes. Each chromosome contains double-stranded molecules, known as DNA, which have a double-helix shape.
Before a cell can divide, it must replicate all of its DNA, producing two identical copies. This process is called DNA replication.
DNA replication begins when the enzyme helicase unwinds the double-helix structure, separating it into two strands.
The Y-shaped region that is formed is called the replication fork, and each branch is made up of a single strand of DNA.
DNA strands have two different ends, the 5 prime (5') end, and the 3 prime (3') end, which show 'directionality'. The two strands that make up a double helix run in opposite directions to each other. The 5' to 3' strand is known as the leading strand, and the 3' to 5' strand is known as the lagging strand.
This orientation determines the direction in which DNA replication will occur.
When the double helix is unwound, each strand has exposed nitrogenous bases that form a template from which the new DNA will be synthesized.
Synthesis of a new strand of DNA on the leading strand requires a primase enzyme to lay down an RNA primer, a short sequence of complementary RNA nucleotides that pair with the leading-strand template. This primer is required because the major enzyme of DNA replication, DNA polymerase, cannot initiate synthesis of a strand of DNA directly from the leading strand alone.
With the primer in place, DNA polymerase is recruited to the RNA and DNA strands and replication begins.
DNA polymerase extends the new strand of DNA from the RNA primer, reading the leading-strand template, and adding complementary free DNA nucleotides to the 3' end of the newly synthesized strand.
These nucleotides have bases which are complementary to those of the leading-strand template: adenine bases bind to thymine bases, and cytosine bases bind to guanine bases.
The DNA polymerase moves continuously along the leading-strand template, synthesizing the new strand in a 5'-3' direction, until replication is complete.
A repair polymerase then replaces the RNA primer with DNA nucleotides.
Because the lagging-strand template is oriented with its 3' end towards the replication fork, synthesis of a new strand of DNA cannot occur continuously. This is because DNA polymerase can only add nucleotides to the 3' end of a DNA molecule.
As with the leading strand, a primase enzyme must lay down an RNA primer, which acts as a marker, signaling where DNA polymerase should be recruited.
The RNA primers are assembled in a 5'-3' direction in short segments.
DNA polymerase constructs the new strand from the primer, reading the lagging-strand template, and adding free DNA nucleotides to the 3' end of the newly synthesized strand, until it reaches a previously assembled primer.
These short strands of DNA are known as Okazaki fragments.
These processes occur multiple times as the DNA is unwound, resulting in short segments of DNA, the Okazaki fragments interspersed with RNA primers.
A repair polymerase then replaces the RNA primers with DNA nucleotides, which are then linked to the fragments by an enzyme known as DNA ligase.
The end product of DNA replication is two double-stranded DNA molecules, both identical to the original one.
